Conventionally, various types of barcodes are used for management of commodities, baggage and so on. Those various types of barcodes include a type of barcode called two-dimensional barcode. In the two-dimensional barcode, information is recorded in extents of two directions, i.e., vertical and horizontal directions, and the two-dimensional barcode has an increased greater memory capacity than a type of one-dimensional barcode in which information is written only in one direction.
In general, a barcode includes a bar having a low reflectance, such as colored in black; and a space having a high reflectance, as a blank part with nothing printed, such as colored in white; while the bar and the space being alternately placed in order to record intended information composed of alphanumeric characters. Sometimes the barcode is used in a form of a barcode label, in which the barcode is printed on a sticker and the like, so as to be attached to an object; or in another way, the barcode is used in a form of being directly printed on an object. While the barcode being scanned with a light beam such as a laser light, reflected light is detected by using a light receiving element of a photo acceptance unit in order to measure width of each bar and each space of the barcode. Then, by way of decoding obtained analog signal, information encoded in the barcode is read.
In a category of two-dimensional barcodes, there is a type in which one-dimensional barcodes are so stacked as to increase its amount of information, and such a type of barcode is called a stack barcode. As a typical example of the stack barcode, there is a coding scheme called PDF417. Incidentally, the coding scheme PDF417 is internationally standardized as ISO/IEC15438.
As an apparatus for reading such a stack barcode, there exists a symbol information reading device, for example, as disclosed in Patent Document 1. The symbol information reading device disclosed in Patent Document 1 images a stack barcode by using a two-dimensional imaging unit, and represents a quality condition of the stack barcode with a numerical value to make it possible to assess a deterioration condition of the stack barcode.